Influence Of Severe Plastic Deformation Methods And Annealing On Cu-Al-Zn Alloys

Severe plastic deformation as the processing methods of ultra-fine crystal/nano materials with high strength and ductility is getting more and more attention in recent years. However, the study of severe plastic deformation mainly focuses on binary alloy materials, the ternary alloy materials involved very few. This paper starts with copper and Cu-Al-Zn alloy, and mainly researches the influence of severe plastic deformation methods and annealing on Cu-Al-Zn alloys.After the processing of high pressure torsion, hopkinson pressure bar, cold-rolling and rolling at room temperature, respectivly, we found that copper and Cu-Al-Zn alloys deformed by severe plastic deformation exhibit better mechanical properties than that deformed by rolling at room temperature. The strain of materials deformed via high pressure torsion process is about8.84, while that deofrmed by rolling at room temperature is only2.98. Therefore, the grain sizes of samples after high pressure torsion processing are smaller than that deformed by rolling at room temperature. According to the Hall-Petch relationship, reducing the smaller grain size may improve the strength and ductility of metals in a certain range. So the high pressure torsion process take advantage of higher strength and ductility comparing with rolling at room temperature process. The strain rate of Hopkinson process is104s-1, which is significantly higher than the samples deformed by rolling at room temperature (5s-1). As Zener-Hollomon parameter (Z value) is proportional to the strain rate, so the samples of Hopkinson process have higher Z value. The increase of Z value will restrain the dynamic response and promote the formation of deformation twin in the process of plastic deformation. Twins can promote the strain hardening, which help delay the onset of necking, thereby promoting the tensile ductility. Consequently, at the same stacking fault energy, the alloys deformed via hopkinson process exhibit better mechanical properties than that deformed by rolling at room temperature. The deformation temperature of the cold-rolling samples is77K, while that of samples deformed by rolling at room temperature is293K. As Z value is inversely proportional to deformation temperature T, and more sensitive to it than strain rate, so the Z values of cold-rolling samples are higher. Consequently, at the same stacking fault energy, the alloys deformed via cold-rolling process exhibit better mechanical properties than that deformed by rolling at room temperature. The results of XRD measurement showed that the dislocation density and the twin density of materials increases with the decreasing of stacking fault energy. These factors working together to improve the strength and ductility of materials in a certain range.The samples deformed by cold-rolling and rolling at room temperature were annealed at different temperature for an hour. The results show that hardening phenomenon appears in Cu-12.1at.%Al-4.1at.%Zn (the stacking fault energy y=7mJ/m2) alloy and Cu-4.3at.%Al-22.8at.%Zn (the stacking fault energy y=10mJ/m2) alloy annealed at150℃and200℃, while at200℃is most obvious. After250℃, the materials begin to soften. As for Cu-2.5at.%Al-2.5at.%Zn (the stacking fault energy γ=40mJ/m2) alloy, there isn’t hardening phenomenon after annealing, so the abnormal phenomenon should have close relationship with the concentration of alloying element. Known from the results of XRD measurement, the grain size increased while the micro-strain and the dislocation density both reduced after annealing.